Bird Survivorship

Are birds immortal? Not exactly. Generally, we consider mortality as immunity from death –living forever. As we all know death is inevitable for all living things, Hymans have extended their life span, though, and some radical thinkers believe immortality is possible.Males born in 1940 were expected to live to be 65, females 70. Today those figures are 72and 84, respectively, due to improved sanitation and medical care, especially vaccinations, clean water and flush toilets. So we age more slowly before we wear out. What about birds? You’ve seen old dogs and cats and other kinds of aged animals, but have you ever seen and old bird? It’s easy for the experienced eye to tell an immature bird from a mature one but can you tell a five-year-old American Robin from a ten-year-old one? I doubt it.

Birds get older, of course, but they don’t seem to age; they don’t even seem to decline slowly. And you rarely see an injured bird. Why is that? Most birds live an active life, using a high metabolic rate to support their flying, foraging, molting, and maybe migrating. You could say they are living on the edge. A few missing feathers, a broken bone, an illness, or other infirmity puts them in serious danger. While a coyote or raccoon can survive with three legs, a bird with one wing cannot. Now, I have seen one-legged birds, and an occasional broken wing, but that’s about the only major injuries I have seen in wild birds. A little science here. There’s a thing called a survivorship curve.

What the survivorship curve indicates is that humans live pretty well for 60 or so years,their chance of dying being equal over that time. After about 60, the chances of dying continually increase. That’s type I. Birds, however, die at about the same rate no matter how old they are. Example: a 65-year-old man is more likely to die in his 66th year than a 30-year-old human in his 31st year. A 10-year-old bird, however, is no more likely to die than a two-year-old bird. That’s Type II. There is a third curve as well and this one applies to many forms of life, especially fish, amphibians, and invertebrates. They might lay thousands of eggs but only a few of them grow to maturity; there is a lot of mortality early on.

Birds walk a fine line and have to be in good shape all the time. Almost any illness orinjury will eliminate a bird from the gene pool. If they stay healthy, avoid breaking a leg orbeing eaten by a fox, they could live many years. There is one caveat. The chances of an egg making it to adulthood the next year are pretty slim; only 10-40% of offspring make it through the egg, nestling, fledgling, and immature stage to get to adulthood, especially if a migratory trip is involved.
See Walt Disney’s It’s Tough to be a Bird for a humorous version of bird survivorship.